This invention is generally directed towards a brake control system, and, in particular, toward a wheel slip detection and correction system for operation in conjunction with a railway propulsion and braking system.
Wheel slip is a recurring problem in the operation of railway cars. Wheel slip occurs when the force applied to a wheel exceeds the frictional forces that hold the wheel in contact with the rail. When the applied force exceeds such frictional forces, the wheel tends to slip. Slip is defined as the wheel's rotation on its axle with relative motion existing between the wheel and the rail at the area of contact. If the slipping condition is not quickly corrected, the wheel may begin to slide or spin. Slide is defined as the wheel not rotating on its axle with relative motion existing between the wheel and the rail when the brakes are applied. Slide is more commonly referred to as wheel lockup. Spinning is the equivalent of sliding during the propulsion mode of operation. Hence, when the propulsion force applied to a wheel is greater than the frictional forces holding the wheel to the rail, the wheels tend to first slip and then spin. Hence, spinning is defined as the wheel rotating on its axle with relative motion existing between the wheel and the rail at the point of contact.
Wheel slip is undesirable for several reasons. It reduces the effective control of the railway car to the extent that the operator is no longer fully in control of the motion of the vehicle. Such a situation is obviously unsafe, and is particularly so during the braking mode of operation. It is believed that the effective braking force of a sliding or slipping wheel is substantially less than the braking force of a wheel that is in normal adhesion with a rail. Hence, when a wheel slips or slides, the braking effort is correspondingly reduced. Such a reduction in braking effort is an obvious hazard. In addition, a sliding wheel tends to acquire flat spots where the wheel slides over the rail. Such flat spots deform the circumference of the wheel. Such deformed wheels are believed to be the case of undesirable vibrations in the railway car. Such vibrations are uncomfortable to passengers and may be damaging to freight. In addition, such deformed wheels ultimately will have to be replaced or resurfaced, at a considerable expense to the railroad.
In the past, there have been a number of attempts to sense and correct wheel slip. These attempts have included mechanical means for sensing a slipping wheel and releasing the brake on the wheel. Such mechanical means have, however, required a considerable amount of maintenace and thus have not been widely accepted. More recently, there have been proposed solid state, electrical analog means for sensing and correcting wheel slip. The disadvantage of such devices, however, is that they too require a considerable amount of maintenance. In particular, one problem of such electrical analog means is the drift inherent in the individual electronic components. A typical analog control circuit for wheel slip requires numerous settings of various bias voltages on different components. In addition, such components have to work in the hostile environment of a railway vehicle and thereby are subject to vibration, heat, cold, dust, and other adverse factors which tend to cause the components to drift from their proper settings. As a result, such systems quickly drift from their desired settings and thereby miscorrect for wheel slip.
Since wheel slip is caused by an excessive force on the wheel, it is usually corrected by removing the applied force from the wheel. Hence, when a wheel begins to slip or slide during braking, the slip can be corrected by momentarily releasing the brakes until the normal frictional forces between the wheel and the rail start the wheel to roll once more. Then the brakes can be reapplied to the wheel. A problem with both the mechanical and the analog electronic systems of the prior art is that their detection and correction for wheel slip was inconsistent and unreliable. Hence, such prior art systems were sensing and correcting for wheel slip when no slipping was occurring. Accordingly, the brakes were being released under normal operating conditions thereby presenting a hazard and a nuisance.
It is believed that many of the problems of the prior art systems arose out of the inability of those systems to compensate for variances in wheel diameters due to the wear and the replacement of the wheels. Obviously, wheels with different diameters will turn at different speeds, even if there is no slipping between the two wheels. Unless such differences can be calibrated into the detection system, then the system will detect a difference in the speed of the two wheels. Such a differential speed will normally be taken by a system as an indication of a slipping wheel.
Another problem with the systems of the prior art has been their inability to detect erroneous readings from axle speed sensors. Hence, if the sensor was generating an erroneous speed signal, the prior art would interpret such an erroneous signal as a slip or a slide.
Another disadvantage of systems of the prior art was their inflexibility to changes. Hence, wheel slip systems of the prior art were generally specially designed for one and only one set of criteria to determine slide or slip. If those criteria were changed, then the entire wheel slip system had to be accordingly modified.